For the purpose of shortening the time required for treatment planning and for reducing the labor of required for treatment planning making, in NPL 1, a technology of predicting a dose volume histogram (DVH) is implemented when a particular irradiation is carried out based on a positional relationship between a target and an organ-at-risk, and a past treatment planning data group. The operator is assisted in the treatment planning by indicating the DVH as a target value in an intensity modulated radiotherapy (IMRT) treatment.